A negative pressure type booster for reducing an operating force during braking of a vehicle is widely known and used. This booster includes a constant pressure chamber always communicating with an intake manifold of an engine so that a negative pressure is applied, a variable pressure chamber selectively under one state in which the variable pressure chamber is cut off from the constant pressure chamber and is communicated with the atmosphere and another state in which the variable pressure chamber is communicated with the constant pressure chamber so that the negative pressure is applied, a vacuum valve for switching the communication between the constant pressure chamber and the variable pressure chamber, and an air valve for switching the communication between the variable pressure chamber and the atmosphere. Through the operation of a brake operation member of a vehicle, the vacuum valve and the air valve are switched so that a pressure difference corresponding to the operating force of the brake operation member is set between the constant pressure chamber and the variable pressure chamber. As a result, an amplified output according to the operating force of the brake operation member is generated.
In recent years, there is known a negative pressure type booster which includes an electrically operated solenoid valve. Even at the time when a driver does not operate the brake operation member, the variable pressure chamber is communicated with the atmosphere by the operation of the solenoid valve so that a large pressure difference is generated between the variable pressure chamber and the constant pressure chamber and a large output is generated. For example, Japanese Patent Unexamined Publication No. Hei. 5-24533 discloses this type of negative pressure type booster.
In a braking fluid pressure control device of a vehicle having braking and wheel control (oversteer restraining control) and the like in which, for example, in the case where a vehicle is put in an oversteering state upon rotation of the vehicle, a pump is driven so that without braking operation by the driver, brake fluid is supplied to the outside front wheel to temporarily increase the brake fluid pressure of the outside front wheel to escape the oversteering state, a front stage pressurizing function for supplying pressurized brake fluid to the pump is required so that the pump can smoothly supply brake fluid to the outside front wheel.
In the case where the front stage pressurizing function is performed by the above conventional negative pressure type booster, for example, in the situation where a vehicle is put in an oversteering state upon rotation of the vehicle, a solenoid valve is first electrically operated without a braking operation of the driver, and a variable pressure chamber is communicated with the atmosphere by the operation of the solenoid valve so that a large pressure difference is generated between the variable pressure chamber and a constant pressure chamber, and the output is generated. Next, the output of the negative pressure type booster is converted into a fluid pressure by a well-known master cylinder, pressurized brake fluid is supplied to a pump, and the pump sucks the pressurized brake fluid and supplies the brake fluid to the side of the outside front wheel.
However, in the structure of fluid pressure piping of a brake apparatus having the braking and wheel control, it has been conceivable to provide a construction such that wheels other than the rotation outside front wheel are communicated with the master cylinder. That is, in the conventional negative pressure type booster, the solenoid valve is operated so that air completely enters the variable pressure chamber and a large output is generated. In the case where the conventional negative pressure type booster as the front stage pressurizing function is operated in the braking and wheel control or the like, a large fluid pressure is directly applied from the master cylinder to wheels other than a wheel to be controlled through the braking and wheel control. Thus, there is a fear that the performance of the braking and wheel control cannot be sufficiently achieved.
Another conventional negative pressure type booster disclosed in Japanese Patent Unexamined Publication No. Hei. 9-226562 includes a housing containing at least one pressure chamber and a movable wall disposed in the housing so as to be capable of moving back and forth. The movable wall divides the pressure chamber into a constant pressure chamber positioned at a front side and in communication with a negative pressure source and a variable pressure chamber positioned at a rear side and selectively communicated with the negative pressure source and the atmosphere. A bottomed cylindrical portion is disposed at the front portion of the housing and extends toward the constant pressure chamber, and a plate member is disposed in the constant pressure chamber. The plate member includes an annular flange portion having a bottomed cylindrical shape and extending outward from an open front end portion, a space portion airtightly separated from the constant pressure chamber and formed between the outer peripheral portion and the bottom portion of the bottomed cylindrical portion, and an opposite portion around the bottomed cylindrical portion of the housing and opposite to the angular flange portion. An introduction passage which is retractable in the axial direction is disposed in the constant pressure chamber, one end of which is held between the bottom portion of the plate member and the bottom portion of the bottomed cylindrical portion of the housing, and communicates the space portion with the variable pressure chamber. A groove is formed at one end portion of the introduction passage and communicates the space portion with the inside of the introduction passage, and a first inlet is formed at the opposite portion of the housing and communicates the constant pressure chamber with the outside of the housing. A second inlet which is formed at the outer peripheral portion of the opposite portion of the housing communicates the constant pressure chamber with the outside of the housing. A first conduit communicates with the first inlet and a second conduit communicates with the second inlet. A switching device communicates the first conduit with the second conduit so that the variable pressure chamber is communicated with the constant pressure chamber through the first conduit, the second conduit, the space portion, the groove, and the inside of the introduction passage, and cuts off the communication between the first conduit and the second conduit to cut off the communication between the variable pressure chamber and the constant pressure chamber so that the first conduit is communicated with the atmosphere and the variable pressure chamber is communicated with the atmosphere through the first conduit, the space portion, the groove, and the inside of the introduction passage. An output member is disposed in the inside of the introduction passage, passes through the bottom of the plate member and the bottomed cylindrical portion, and outputs a propelling force to the outside of the apparatus by the movement of the movable wall.
In this conventional negative pressure type booster, when the switching device is operated to communicate the space portion with the atmosphere through the first conduit, the air flowing into the space portion flows into the inside of the introduction passage from the space portion through the groove formed on the one end portion of the introduction passage, and then flows into the variable pressure chamber. When the air is introduced into the variable pressure chamber, a pressure difference is generated between the constant pressure chamber and the variable pressure chamber, the movable wall is advanced, and the propelling force caused by the advance of the movable wall is outputted to the outside of the apparatus through the output member.
However, in the foregoing conventional negative pressure type booster, when air flows into the inside of the introduction passage from the space portion, the air must pass through the thin groove formed at the one end portion of the introduction passage and so there is a fear that the introduction of the air from the space portion to the constant pressure chamber as well as the introduction passage cannot be smoothly carried out.
Moreover, although the one end portion of the introduction passage is held between the bottomed cylindrical portion of the housing and the bottom portion of the plate member, some negative pressure type boosters are structured such that a portion bulging in a constant pressure chamber is made a cylindrical portion and a rear portion of a master cylinder is disposed to the inside of the constant pressure chamber through this cylindrical portion. In the negative pressure type booster of such a construction, there is a fear that such a disadvantage arises that one end portion of an introduction passage is not stably held.
Moreover, because the rear end portion of the master cylinder is disposed at the bottomed cylindrical portion of the housing, a reinforcing plate having a substantially bottomed cylindrical shape is disposed around the bottomed cylindrical portion of the housing of the negative pressure type booster to reinforce the housing. However, in the foregoing conventional negative pressure type booster, because the space portion is formed also between the outside flange portion of the plate member and the portion opposite to the outer flange portion of the housing, when the reinforcing plate is set in the conventional negative pressure type booster, a large space is required for the formation of the space portion at the front portion of the constant pressure chamber so that the volume of the constant pressure chamber is decreased and the use of space in the constant pressure chamber is hindered.
Moreover, the switching device is a member separate from the housing and is disposed outside of the housing. This increases the size of the negative pressure type booster.
In light of the foregoing, a need exists for a negative pressure type booster which can be automatically operated and can be applied for a front stage pressurizing function in a brake fluid pressure control apparatus with braking and steering control and the like, and which makes the braking and steering control and the like more effective.
A need also exists for a negative pressure type booster in which the inflow of air from a space portion into a variable pressure chamber can be smoothly carried out.